Many processes involve the use of large bodies of liquid where the temperature of the liquid is controlled to maximize efficiency of the processes. In order to maintain a substantially uniform optimum temperature throughout the liquid, continued agitation of the liquid is required. Specifically, where solid material is present in the liquid, agitation of the liquid further provides a more uniform suspension of the solids dispersed in the liquid. These requirements are present in the anaerobic digestion system for waste water containing biosolids.
The process of anaerobically digesting municipal and industrial waste waters has been in practice for many years. The process involves the collection of waste water and sludge into large sealed digesters or holding tanks. The material is typically derived from raw sewage containing about ninety five percent (95%) liquid-type material and about five percent (5%) solid-type material. Microbes are introduced into the digester tank to feed and decompose the sludge into other byproducts. After the digestion process is completed, the digested sludge is safely deposited into landfills or recycled as fertilizer.
Early use of unheated and unmixed digesters proved the effectiveness of anaerobic digestion. However, it was soon discovered that heating the contents of the anaerobic digester to approximately 35 degrees C..degree. (95 degrees F..degree.), would increase the rate of digestion. This improvement was capitalized in the form of reduction in digester size, higher loading rates and shorter detention time. With thorough heating of the digester contents to a relatively uniform level, an even higher rate of digestion was achieved. When uniform digester temperature and uniform suspension of solids were maintained, through continuous mixing of the digester contents, the digestion process was additionally efficient.
Generally, mixing systems for digesting waste water should produce a ninety percent (90%) active digester volume. That is, at least ninety percent (90%) of the volume of the digester is active and available for microbes to feed and digest the sludge. Many different types of mixing systems have been developed over the years for the anaerobic digestion process.
Specifically, mixing systems were developed that recirculate digester gas through draft tubes, and that mixing serves to main a more uniform digester temperature and increases the active digester volume. In a gas mixing system, a liquid circulation device composed of a draft tube and a "piston bubble" generator is submerged within the contents of the digester.
Each digester tank may incorporate a number of these liquid circulation devices. The quantity and size of the draft tubes depend upon the size of the digester and the specific characteristics of the waste to be digested.
The draft tube is an open-ended cylinder typically, that is placed vertically within the digester. Recirculated digester gas is piped to a bubble generator from the gas dome of the digester cover by a compressor. Gas bubbles are generated and then introduced into the lower portion of the liquid-filled draft tube, The gas bubble propels the material up and through the upper end of the draft tube. The gas rises to the digester tank, where it is collected and piped back to the bubble generator.
Preferably, each gas bubble released into the draft tube will expand freely and fill the diameter of the draft tube. The expanded bubble rising up the draft tube functions like a piston that forces the liquid in front of the bubble out of the draft tube while drawing liquid by back pressure into the lower end of the draft tube. The effect of a bubble leaving the top of the draft tube as the next bubble enters the draft tube creates a positive liquid flow that moves a volume of liquid through the tube between successive bubbles. This continuous and cyclic movement of bubbles inside the draft tube facilitates continuous circulation and mixing of the liquid and suspended solid material from the bottom of the digester tank up to near the top of the contents.
The wide acceptance of this piston bubble design of mixing system proves its effectiveness and reliability. The key to a successful design of such a system is a means for efficiently creating a series of bubbles of controlled size and frequency, which fill the draft tube. No moving parts in the digester are preferred. Furthermore, the design must safeguard against the problem of clogging from the suspended solid material, so that the mixing system functions with little or no maintenance.
An early design of a gas mixing system placed a bubble generator below a draft tube, as in Lipert U.S. Pat. No. 4,169,873. This initial design successfully demonstrates the efficiency of piston bubbles effecting liquid circulation through a draft tube. However, this design is prone to the problem of blockage from strands or clumps of solid material catching in the tight inlet between the bubble generator and the draft tube. This continuous flow of liquid with suspended solid material around the bubble generator greatly increased the risk of clogging the system. This problem was increased by the placement in the lower part of the tank where the volume of settling solids was greatest. Also, liquid and solids being pulled up the draft tube must pass by the bubble generator. The bubble generator being placed below the draft tube in the flow path was a barrier that inhibited the flow of liquid into the draft tube, thereby reducing the pumping efficiency of the system.
Another design of gas mixing system which was useful for circulating fluids composed of long chain molecules is the Lipert U.S. Pat. No. 4,356,131, which discloses a bubble generator that introduces bubbles through the side of a draft tube. A pipe connected to the side of the stackpipe extends outwardly forming a T, then it curves down toward its termination at a flared downwardly pointing frusto-conical opening. Beneath that opening is a stand pipe that is connected to a gas accumulator tank via a bent pipe, and at that connection, gas is introduced. The placement of the bubble generator on the side reduces the problem of having the draft tube being partially obstructed by the bubble generator. Side placement, however, creates bubbles that travel up the side, rather than the center, of the draft tube. Also, if clogging occurs at the inlet to the draft tube from the side-pipe, then bubbles may be impeded from entering the draft tube or may be deformed as they enter the draft tube.
As between mounting the bubble generator below the draft tube or on its side, one effect is upon how soon the bubble forms to fill the area of the tube. In the side mounted design, the bubble tends to move upward more rapidly than it expands laterally to fill the tube. Until the bubble fills the tube, no real draft pressure is present within the tube. To deal with this, longer draft tubes were needed, or lower mounted bubble generators were used. Longer tubes require larger tanks. Lower placement, where more solids are present, increases clogging and increases the pressure levels needed to circulate the solids-laden liquid.
Therefore, a need exists in the art for a liquid circulation device that generates rapidly-forming piston bubbles that fill the draft tube to circulate liquid within a digester without the use of moving parts and that possesses safeguards against clogging from solid material suspended in the liquid.